The butene-1--isobutene separation problem has existed for a long time. A variety of solutions have been proposed but none is completely satisfactory in terms of efficiency of separation without the appearance of by-product(s). U.S. Pat. No. 2,403,672 describes a process for separating isobutene from an isobutene--butene-1 mixture which comprises introducing the mixture into an isomerisation zone and fractionating it, wherein the isomerisation catalyst also acts as a packing to allow distillation. This solution has the major disadvantage of not being very efficient in distillation and thus has a mediocre capacity for separating isobutene from butene-1.
The process of the invention can produce high purity isobutene, at low cost and in a high yield, from an olefinic C.sub.4 cut containing at least isobutene, as well as butene-1 and butene-2 compounds, in a ratio which substantially corresponds to the thermodynamic equilibrium, the cut generally originating from a steam cracking process, such as a crude C.sub.4 cut or 1-raffinate (obtained after extracting butadiene from the crude C.sub.4 cut), or from catalytic cracking. The process of the invention is characterized in that the operations of distillation, hydroisomerisation and skeletal isomerisation of the linear butenes to isobutene are integrated, and arranged and operated to minimise the investment costs of the process, to maximise conversion of butene-1 to butene-2 compounds, to minimise hydrogenation of isobutene to isobutane and to transform a portion of the linear butenes to isobutene in order to maximise the isobutene yield. Thus the process of the invention can effect at least partial selective hydrogenation of polyunsaturated compounds which are usually dienes or acetylenes such as butadiene, vinylacetylene, methylacetylene and ethylacetylene, when these compounds are present in the feed, hydroisomerisation of butene-1 to butene-2 compounds (cis and trans), and skeletal isomerisation of linear butenes to isobutene. The butene-2 compounds produced from this hydrogenation and hydroisomerisation can then be separated from the isobutene by distillation, which is not the case with butene-1. The bottom product from the reactive distillation zone, which contains mainly butene-2 compounds and possibly normal-butane when this is present in the feed, passes into a skeletal isomerisation unit where at least part of the linear butenes is transformed into isobutene; the products from this skeletal isomerisation are recycled upstream of the reactive distillation step, for example mixed with the feed from the reactive distillation zone, in order to separate the isobutene produced from the linear butenes. Compared with the other processes cited above, the process of the invention has the advantage of having a very high isobutene yield which is generally over 100% if linear butenes are present in the feed. When the feed is a C.sub.4 cut from a steam cracking unit from which the major portion of the butadiene has been extracted, i.e. a 1-raffinate, the yields are generally over 120%.